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Glass Blowing for Vacuum Devices

Blowing Bulbs

To blow a bulb on the end of a tube, it is first necessary to obtain a sealed tube end. If the bulb is to be employed for vacuum work, the seal must be gas tight. It is possible to seal the end of the tube by rotating the tube in a high flame at a steep angle, as in rounding the end of a tube as described previously, but continuing until the end of the tube collapses. I have found that this method is very prone to leaving a small pin hole in the end of the tube. It can be very difficult to get it to fully seal.

I had to stop at times to take photographs, but it is best to work through all of the following stages without stopping. This avoids having to keep heating the glass back up from room temperature and reduces the risk of cracking.

Starting with a 330mm long 12mm diameter tube, I introduce a point 80mm from one end to a medium flame. To reduce waste I sometimes start with a 250mm length and join it to a scrap piece of tube with the same diameter. I rotate the tube in the flame using both hands.

The lower surface of the tube should be just above the bright blue part of the flame. This is the hottest part of the flame. I continue rotating the tube in the flame until it begins to soften.

When the glass softens, the flame turns orange and then yellow. I then draw the two halves of the tube apart. The tube thins in the middle as can be seen in the picture. It is important not to draw the two halves apart too fast or a long cone shaped end results. I try to get the end of the longer part of the tube to collapse without extending very much. I keep the middle part in the flame so that it collapses and seals. Finally, I draw the two halves completely apart.

Excess glass can be drawn away using another piece of glass. I keep repeating this until the length of the cone is less than the diameter of the tube, but more than the radius. The result of this can be seen in the next picture.

This is how it looks after drawing off the excess glass. The end of the tube should still be completely sealed and should not be significantly thinner than the wall thickness.

It is essential that the open end is rounded over so that there are no sharp edges, as it is put in the mouth to blow the bulb. How this is done is described on the Cutting Tubes page.

I rotate the end of the tube in the flame at an angle using both hands. I then remove it from the flame, put the open end in my mouth and blow, gently at first and then harder. The aim is to get the end round like a test tube bottom. It is safest to do this in several stages with the glass only just as hot as required. I heat it a little more each time to find the correct temperature.

It is important not to blow while the end of the tube is still in the flame. If this is done, it will be hotter on one side and will blow out suddenly and unevenly.

This is how it looks after blowing the end round. The end should be completely smooth.

The bulb can not be blown to full size in one go, or it would end up too thin. It must be started in stages.

The end of the tube is then heated again, while rotating it all the time in a horizontal position. I lick my lips so that the tube will rotate in my mouth. I remove the tube from the flame and put the open end in my mouth. I keep the tube rotating and blow gently at first. I puff my cheeks out so as to get more control. If it is not hot enough, I return it to the flame and try again. It is best to have the glass just hot enough to work and it is important not to blow while the tube is in the flame. I blow a small bulb on the end as shown in the picture.

I then repeat the process just next to the first bulb. I blow out a second bulb of a similar diameter to the first. It must be rotated faster now because there is more weight on the softened section. I then heat both bulbs and blow gently, allowing them to merge together.

This is the result of the previous step. It will be blown out into a spherical bulb at this stage, but a larger bulb could be made by blowing a third bulb next to the first two. If I were using a burner with a smaller flame, it may be necessary to do this anyway.

I heat the expanded section as evenly as I can. The tube must be rotated very fast now or the end will sag. I only need to blow gently now because it is so much thinner. It is important to keep it rotating even while blowing. I repeat this stage until a perfectly spherical bulb is obtained. On the final blowing stage, I gradually blow harder as the glass hardens. I find that this helps to produce a more spherical bulb.

This is the completed bulb. It is 42mm in diameter and has an even wall thickness. blowing in three stages rather than two produces a bulb which is about 50mm in diameter using my burner.

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